for UPSC IES and GATE 2014 Mechanical Engineering
The branch of science that deals with the relations
between heat and other forms of energy, and of
the relationships between all forms of energy.
System and Surroundings
• A system is a ﬁnite quantity of matter or a
prescribed region of space.
• The actual or hypothetical envelope
enclosing the system is called Boundary.
• Boundary may be ﬁxed or moveable.
• Everything external to the system is
• A System and its Surroundings together
comprise a Universe.
Types of System
• Open System- In this matter and
energy ﬂow in or out of the
system. Most of the Engineering
Systems are Open Systems.
• Closed System- In this the
boundary is impervious to the
ﬂow of matter. A mass of gas and
vapour in an engine cylinder, with
a continuous boundary may be
regarded as a Closed System.
• Isolated System- In this the
system neither exchanges energy
nor matter with another system
Adiabatic System vs Isolated System
• An adiabatic system is one which is thermally insulated
from its surroundings.
• This means no heat exchange across system boundary.
• Work transfer can take place across the system boundary.
• If work transfer does not occur across the boundary, it
becomes an ISOLATED SYSTEM.
Macroscopic and Microscopic Points of View
Thermodynamic studies are undertaken by the two different
approaches:• Macroscopic approach - means Big or Total.
• Microscopic approach - means small
Although both approaches are different, but when applied
to a system, both provide same results.
Properties, Processes and Cycles
• When properties like, volume, pressure, temperature etc. of
a system have deﬁnite values, then the system is said to
exist at a deﬁnite State.
• Properties are the coordinates which describe the state of a
• An operation in which one or more properties of a system
changes is called change of state.
• The succession of States passed during a change of state is
called a process.
• A cycle is deﬁned as a series of state changes in which
the initial and ﬁnal states are identical.
• Two types of properties:- Intensive properties, which
are independent of the mass of the system; e.g.
Pressure, Temperature etc. Extensive properties, are
related to mass; e.g. Volume, Energy etc.
• If mass is increased, the values of extensive properties
• Speciﬁc extensive properties, i.e. Extensive properties
per unit mass, are intensive properties, e.g. Speciﬁc
• Thermodynamic equilibrium exists in a system when no change of
macroscopic properties is registered, if the system is isolated from
• An isolated system always reaches in course of time a state of
thermodynamic equilibrium and can never depart from it
• If a system is in equilibrium, no spontaneous change in
macroscopic property can occur.
• Thermodynamic equilibrium exists when, Mechanical, Thermal and
Chemical equilibrium exist simultaneously.
Zeroth Law of Thermodynamics
When a body A is in thermal equilibrium with a body B, and
also separately with a body C, then B and C will be in thermal
equilibrium with each other.
• Zeroth law is the basis of
• There are ﬁve kinds of
thermometer, each with its own
Constant vol. gas
Constant pressure gas
Mercury in glass
• Work is done by a force as it acts upon a body moving in
the direction of the force.
• When work is done by a system, it is taken to be positive,
and if work is done on a system, it is taken to be negative.
• Work is one of the forms in which a system and its
• When the piston moves, the
volume changes, and the work
done can be calculated as
described in the ﬁgure.
• The magnitude of the work done
is the area under the P-V diagram.
• The path shown on the P-V
diagram must pass through
equilibrium states; i.e. must be
• The integration to calculate the
work done can be performed only
if the process is quasi static.
Path Function and Point Function
• Area under the PV curve gives the amount of work done,
so it is not a function of the end states of a process, but
depends upon the path the process takes. Therefore,
work is called a path function.
• Thermodynamic properties are point functions, since for a
given state, there is a deﬁnite value for each property.
• Path Functions are called inexact or imperfect differential.
• Point Functions are called exact or perfect differential.
Work in various Quasi
PVk = constant
Open and closed
• The open system work is
calculated by plotting it on
• The closed system work is
calculated by plotting it on
Zero work transfer
• Work transfer is ONLY identiﬁed at the boundaries of a
system. Therefore, it can be termed as a boundary
• The expansion of a gas against vacuum is called free
expansion. In this case the work done by the system is
zero, as no work crosses the boundary.
• This free expansion process is not a quasi static process,
although the initial and end states are in equilibrium.
• It is deﬁned as a form of energy that is transferred across
a boundary due to a temperature difference.
• The direction of heat transfer is from a body at high
temperature to a body at lower temperature.
• Heat ﬂow into a system is taken to be positive, and heat
ﬂow out of a system is taken to be negative.
• Heat transfer is a transit, and occurs only at the boundary.
Important points for a Heat and Work Transfer
Heat and work transfer are energy interactions.
The same eﬀect in a closed system can be brought
about either by heat transfer or work transfer.
Both transfers are boundary phenomena, and both
represent energy crossing the boundaries of the system.
It is wrong to say 'total heat' of a closed system as heat and work
are not a property of a system. They cannot be stored in the
system. Both are energies in transit
Heat transfer is the energy interaction due to temperature diﬀerence
only. All other energy interactions can be termed as work transfer
Next topic:- First Law of Thermodynamics
For any doubts regarding this lecture feel free to drop me an
email at, firstname.lastname@example.org.